1. Field of the Invention
This invention relates to polymeric films and more particularly, to a novel process for manufacturing very thin freestanding polymeric films.
2. Description of Related Art
Very thin, freestanding polymeric films have a multitude of uses in today's world, including packaging, membranes, dielectric films, and diffusion release control for drug applications. Although films can be coated onto other substrates which are nanometers in thickness, these are not freestanding films, since they cannot be peeled off the substrate and handled in a robust way.
There are currently several manufacturing methods for making very thin freestanding films in commercial quantities. Each genre of manufacturing techniques has distinct drawbacks and disadvantages with respect to thin freestanding polymeric films.
Blown films are made by melting the polymer using an extruder, forcing the polymer melt through a vertical annular die, and keeping the resulting cylindrical column of polymer from collapsing by using high pressure air in the middle of the annulus. This air also cools the polymer, causing it to solidify into a flexible, polymeric annulus. The annulus is then slit and wound to form a roll of film. This technique is frequently used to make grocery bags out of high density polyethylene and other materials. It is fairly inexpensive, and capital costs are low compared to other methods. The disadvantage of this technique is that it is primarily limited to a narrow range of olefinic polymers such as polyethylene, and the thickness variability of the resulting film is often poor. The polymers must be “shear thinning” to be economical, since Newtonian polymers which do not have reduced viscosity under the high shear conditions of the annular die will have a very high pressure drop associated with them. This high pressure can damage the equipment and limit the productivity of the extrusion operation. The blown film process is also limited in the degree of filler such as amorphous silica or carbon black which can be used. The presence of filler usually dramatically increases the viscosity (and therefore the pressure drop) of the melt. This filler also reduces the melt strength, resulting in instability of the vertical column of polymer and causing repeated breaks. This would limit the minimum thickness of the films made with such a process, as well as the productivity and economics derived.
Chemically cross linked polymers cannot be used for the same reason that high fillers cannot be incorporated, in particular, they would cause an exceedingly high pressure drop. In addition to equipment limitations, the high pressure drop which would result causes a phenomenon called “melt fracture” through the die. Melt fracture results in very poor extruded quality.
Another well established technique used in the manufacture of films is the cast film process. In the cast film process, the polymer is again melted in an extruder, and forced through a rectangular die (rather than an annular die used in blown film). The rectangular die is usually a coat hanger, horseshoe or T type die, where the resulting polymer melt is subsequently cooled on chill rollers and then wound to form a plastic roll. Like the blown film technology, this technique has a relatively low capital cost, and can be used on a slightly wider array of materials. A depiction of the prior art cast film process is shown in FIG. 1, where the film is low density polyethylene (LDPE). Cast film gives a film with much better thickness uniformity than blown film. The disadvantage of the cast film process is that, like blown film, the thickness and speed of the extruded materials made using this technique depends on the melt strength of the thermoplastic which is being extruded. There are many valuable thermoplastics such as polycarbonate, polymethyl methacrylate, and other amorphous polymers which do not have good melt strength. As used in this specification, an amorphous plastic means a polymer having less than 5% crystallinity as measured by differential scanning calorimetry. If an attempt is made to extrude these amorphous thermoplastics as a thin film, the result will be as shown in FIG. 2, where the melt curtain has broken and the film making process interrupted. Thus, the cast film process works adequately when trying to make thick films from amorphous polymers (>10 um), but it works poorly when making very thin films. In addition, when highly loaded films such as films with fillers like carbon nanotubes, or silica particles are desired, the cast film processes is again limited because the addition of these materials also contributes to the reduction in melt strength. Therefore the cast film process for making highly loaded films is limited to making thick films or sheets. Highly cross linked polymers also cannot be processed in cast films for the same reasons as described with respect to blown films, namely exceedingly high pressure and melt fracture.
A third well established technique uses solvent coating. In this method, a polymer is dissolved in a suitable solvent, and then cast through a rectangular coat hanger, horseshoe die T die or X-hopper (depending on the viscosity), and coated onto a surface such as a large roll or band where the solvent is subsequently evaporated. When enough of the solvent has been evaporated, the web can be peeled off the roll or band, dried further, and then wound into roll form. This process has been used for decades to make thick cellulose triacetate films. The advantage of this process is that it can be used to make very thin films of some amorphous plastics (depending on solubility, such as polycarbonate in trichloromethane) and it can be further used to process some non-thermoplastic polymers such as unplasticized polyvinyl alcohol. The disadvantages of this approach are (1) high capital cost for equipment necessary to handle the solvents and for drying the solvents, (2) many of the solvents used for dissolving polymers are toxic or carcinogenic, (3) the process is slow due to diffusion control of the drying process, (4) it is very difficult to remove the last 1% or so of the solvent, which results in an impurity which may not be desired, and (5) some plastics are not even soluble in commonly available solvents or solvent combinations. In addition, films highly loaded with fillers cannot be made unless dispersing agents are added to the filler to prevent premature precipitation. This dispersant is often an undesirable impurity.
Another well established technique to make thin freestanding films uses biaxial orientation of a thick sheet. In this technique, the polymer is melted in an extruder, and forced through a slot die and onto a chilled roller, as with the cast film process. This produces a thick sheet of the polymer. The polymer is then reheated and biaxially oriented, usually in a two step process, drafting the sheet in the machine direction first, which increases the machine direction speed and reduces the thickness, and then by tentering in the cross direction, increasing the width and further decreasing the thickness of the film. This method is frequently used to great advantage in making polyester films from polyethylene terephthalate and polypropylene films in the BOPP (biaxially oriented polypropylene) process for food wrappers or garbage bags. The advantage is that it can be used to make exceedingly thin films (of crystalline and semi crystalline polymers), often in the one micron range. It can also be used for highly filled webs, but only if voiding of the film is desired. The disadvantages are (1) high capital cost, often in the $100 M range, and (2) required use of crystalline polymers or semi crystalline polymers like polyethylene terephthalate and polypropylene to be of good advantage. The process cannot be readily adapted to significantly biax amorphous polymers like acrylic or polycarbonate, and (3) it is difficult to use cross linked polymers since these will not orient biaxially without tearing the web.
The above mentioned approaches are all widely used and are firmly entrenched commercial manufacturing techniques. However, all of these methods have difficulty in economically making some important types of freestanding films. These types of films include: very thin freestanding films made from amorphous polymers like polycarbonate, polymethyl methacrylate, polysulfone and others; highly filled films made from amorphous, semi crystalline, or crystalline polymers; and chemically cross linked films. A method of manufacturing these film types would be highly desirable.
Very thin freestanding films made from amorphous polymers like polycarbonate, polymethyl methacrylate, polysulfone and others would be useful for dielectric films, protective films or optics films to provide an excellent gloss, membranes, or other uses. Highly filled films made from amorphous, semi crystalline, or crystalline polymers would be useful for making conductive films, highly pigmented films, or films filled with reinforcing fillers. Chemically cross linked films would provide a technique for making insoluble and durable films such as epoxy films or modified polyvinyl alcohol films useful for ethanol pervaporation membranes.